The present invention relates to the transmission of digital data in general and more particularly to the compression of source information which is or can be presented as a bi-level signal train. The invention was made pursuant to development of methods and systems for the transmission of facsimile data but is not limited thereto.
In the earlier phases of development of systems for transmission of facsimile data, one preceded usually by transmitting the scanned video information as a line scan signal which modulated a carrier. The same is true with regard to telemetric information. Since a considerable amount of facsimile data represents nothing but white space, various methods of data compression have been developed with the particular goal in mind to shorten the transmission time. Particularly here various schemes have been developed to avoid the transmission e.g. of long runs of white as video information. These data compression schemes are all based on statistical evaluation of video data. If one interprets video data as bi-level signals corresponding to dark and light contrasts digitization of a train of such signals will result in bi-valued bits respectively representing black and white image increments. The various data compression schemes replace such bits or sequences of bits with others, identifying the video information differently and, hopefully, in a manner that leads to a shorter length of a bit stream to be transmitted as compared with just straight forward transmission of image point by image point.